A solid type rubber-based pressure-sensitive adhesive composition is prepared by using a rubber as a main polymer, and mixing a tackifier and the like therewith, followed by kneading to give a solid type rubber-based pressure-sensitive adhesive composition, and it is applied on a substrate to produce a pressure-sensitive adhesive sheet.
The solid type rubber-based pressure-sensitive adhesive composition which is used in this process is a solventless and nonaqueous pressure-sensitive adhesive composition using neither any organic solvent which causes environmental pollution nor water which necessitates much energy for drying. Usually, a rubbery polymer including natural rubber is used as a base compound, a tackifier is mixed therewith, and a filler such as a fine calcium carbonate powder, a softener such as an oil, an antioxidant and the like are mixed therewith in appropriate amounts. This mixture is kneaded by a batch-type kneader such as a pressure kneader, a Banbury mixer or mixing roll, or a continuous kneader such as a twin-screw extruder, with heating to obtain a pressure-sensitive adhesive composition which is solid at ordinary temperature.
However, although nonpolar rubbers such as a natural rubber, a butyl rubber or a styrene-butadiene rubber have often been used in the rubber-based pressure-sensitive adhesive composition and they have an excellent adhesive performance, they are unusable for heat-resistant uses at 100° C. or higher, resulting in a failure to pioneer the heat-resistant uses.
On the other hand, patent documents 1 and 2 describe that for a pressure-sensitive adhesive for surface protective film, an acrylic rubber is used as a base compound, and that a methylol compound as a crosslinking agent or a methylol compound and a multifunctional isocyanate as crosslinking agents are mixed therewith to allow the above-mentioned crosslinking agent(s) to conduct a crosslinking reaction with functional groups allowed to be contained in the acrylic rubber, thereby achieving an effect of improving weather resistance of the pressure-sensitive adhesive or inhibiting deterioration with time of adhesive strength after storage at high temperatures for a long period of time.
Patent Document 1: Japanese Patent No. 2600360
Patent Document 2: Japanese Patent No. 2600361